“Stay Back” is meaningless. Especially as it is appearing on other vehicles which don’t have big blindspots:
“Stay Back” isn’t the best wording, which is the point of the OP. It’s being redesigned, but in the meantime it’s the best we’ve got
For cyclists from other countries, of which there are many in most cities (students, etc), “Stay Back” is quickly understandable without a phrase book (blind spots and undertakers will probably be in the phrase book section on health)
“If you can’t see my mirrors…” doesn’t work for the type of accidents that we’re discussing. I need to be able to see you
Too wordy and too complex is easily misread, misunderstood or plain misleading
I agree that it shouldn’t be on small vans and cars, it devalues the message that 20% of cycling fatalities in London involve LGVs, so be careful around large vehicles, but I don’t see it as reducing a driver’s responsibilities (or shifting blame to cyclists). If I made a sticker saying, “Stay Back, I’m not insured” I wouldn’t expect the Courts to back me and make your insurance company pay when I drove into you
To summarise IMHO, “Stay Back” is the best we’ve got just now
A form of words/symbols that’s agreed by most, understood by most and used appropriately is going to be some way in the distant future while cyclists are dying in the present